CLASSIFICATION OF SOLID PARTICLES
The problem of separating solid particles according to their physical properties is of great
importance with large-scale operations in the mining industry, where it is necessary to separate
the valuable constituents in a mineral from the adhering gangue, as it is called, which is usually
of a lower density. In this case, it is first necessary to crush the material so that each individual
particle contains only one constituent. There is a similar problem in coal washing plants in
which dirt is separated from the clean coal. The processing industries are more usually
concerned with separating a single material, such as the product from a size reduction plant,
into a number of size fractions, or in obtaining a uniform material for incorporation in a system
in which a chemical reaction takes place. As similar problems are involved in separating a
mixture into its constituents and into size fractions, the two processes are considered together.
Separation depends on the selection of a process in which the behaviour of the material is
influenced to a very marked degree by some physical property. Thus, if a material is to be
separated into various size fractions, a sieving method may be used because this process
depends primarily on the size of the particles, though other physical properties such as the
shape of the particles and their tendency to agglomerate may also be involved. In general, large
particles are separated into size fractions by means of screens, and small particles, which would
clog the fine apertures of the screen or for which it would be impracticable to make the
openings sufficiently fine, are separated in a fluid. Fluid separation is commonly used for
separating a mixture of two materials though magnetic, electrostatic and froth flotation methods
are also used where appropriate.
PARTICLE SIZE REDUCTION
Materials are rarely found in the size range required, and it is often necessary either to decrease
or to increase the particle size. When, for example, the starting material is too coarse, and
possibly in the form of large rocks, and the final product needs to be a fine powder, the particle
size will have to be progressively reduced in stages. The most appropriate type of machine at
each stage depends, not only on the size of the feed and of the product, but also on such
properties as compressive strength, brittleness and stickiness. For example, the first stage in
the process may require the use of a large jaw crusher and the final stage a sand grinder, two
machines of very different characters. In the materials processing industry, size reduction or
comminution is usually carried out in order to increase the surface area because, in most
reactions involving solid particles, the rate of reactions is directly proportional to the area of
contact with a second phase. Thus the rate of combustion of solid particles is proportional to
the area presented to the gas, though a number of secondary factors may also be involved. For
example, the free flow of gas may be impeded because of the higher resistance to flow of a bed
of small particles. In leaching, not only is the rate of extraction increased by virtue of the
increased area of contact between the solvent and the solid, but the distance the solvent has to
penetrate into the particles in order to gain access to the more remote pockets of solute is also
reduced. This factor is also important in the drying of porous solids, where reduction in size
causes both an increase in area and a reduction in the distance. the moisture must travel within
the particles in order to reach the surface. In this case, the capillary forces acting on the moisture
are also affected. There are a number of other reasons for carrying out size reduction. It may,
for example, be necessary to break a material into very small particles in order to separate two
constituents, especially where one is dispersed in small isolated pockets. In addition, the
properties of a material may be considerably influenced by the particle size and, for example,
the chemical reactivity of fine particles is greater than that of coarse particles, and the colour
and covering power of a pigment is considerably affected by the size of the particles. In
addition, far more intimate mixing of solids can be achieved if the particle size is small. The
energy required to effect size reduction is related to the internal structure of the material and
the process consists of two parts, first opening up any small fissures which are already present,
and secondly forming new surface. A material such as coal contains a number of small cracks
and tends first to break along these, and therefore the large pieces are broken up more readily
than the small ones. Since a very much greater increase in surface results from crushing a given
quantity of fine as opposed to coarse material, fine grinding requires very much more power.
Methods of operating crushers
There are two distinct methods of feeding material to a crusher. The first, known as free
crushing, involves feeding the material at a comparatively low rate so that the product can
readily escape. Its residence time in the machine is therefore short and the production of
appreciable quantities of undersize material is avoided. The second method is known as choke
feeding. In this case, the machine is kept full of material and discharge of the product is
impeded so that the material remains in the crusher for a longer period. This results in a higher
degree of crushing, although the capacity of the machine is reduced and energy consumption
is high because of the cushioning action produced by the accumulated product. This method is
therefore used only when a comparatively small amount of materials is to be crushed and when
it is desired to complete the whole of the size reduction in one operation. If the plant is operated,
as in choke feeding, so that the material is passed only once through the equipment, the process
is known as open circuit grinding. If, on the other hand, the product contains material which is
insufficiently crushed, it may be necessary to separate the product and return the oversize
material for a second crushing. This system which is generally to be preferred, is known as
closed circuit grinding.
The equipment may also be classified, to some extent, according to the nature of the force
which is applied though, as a number of forces are generally involved, it is a less convenient
basis. Grinding may be carried out either wet or dry, although wet grinding is generally
applicable only with low speed mills. The advantages of wet grinding are:
(a) The power consumption is reduced by about 20–30 per cent.
(b) The capacity of the plant is increased.
(c) The removal of the product is facilitated and the amount of fines is reduced.
(d) Dust formation is eliminated.
(e) The solids are more easily handled.
The choice of a machine for a given crushing operation is influenced by the nature of the
product required and the quantity and size of material to be handled. The more important
properties of the feed apart from its size are as follows:
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Hardness-The hardness of the material affects the power consumption and the wear on
the machine. With hard and abrasive materials, it is necessary to use a low-speed
machine and to protect the bearings from the abrasive dusts that are produced. Pressure
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lubrication is recommended. Materials are arranged in order of increasing hardness in
the Mohr scale in which the first four items rank as soft and the remainder as hard.
Structure- Normal granular materials such as coal, ores and rocks can be effectively
crushed employing the normal forces of compression, impact, and so on. With fibrous
materials a tearing action is required.
Moisture content-It is found that materials do not flow well if they contain between
about 5 and 50 per cent of moisture. Under these conditions the material tends to cake
together in the form of balls. In general, grinding can be carried out satisfactorily
outside these limits.
Crushing strength- The power required for crushing is almost directly proportional to
the crushing strength of the material.
Friability. The friability of the material is its tendency to fracture during normal
handling. In general, a crystalline material will break along well-defined planes and the
power required for crushing will increase as the particle size is reduced.
Stickiness. A sticky material will tend to clog the grinding equipment and it should
therefore be ground in a plant that can be cleaned easily.
Soapiness. In general, this is a measure of the coefficient of friction of the surface of
the material. If the coefficient of friction is low, the crushing may be more difficult.
Explosive materials must be ground wet or in the presence of an inert atmosphere.
Materials yielding dusts that are harmful to the health must be ground under conditions
where the dust is not allowed to escape.
SEDIMENTATION
A number of mechanical separations are influenced by the sedimentation of solid particles or
liquid drops through a fluid, impelled by the force of gravity or by centrifugal force. In some
cases, the aim of the sedimentation process is to remove particles from a stream in order to
eliminate contaminants from the fluid or to recover the particles, as in the elimination of dust
and fumes from air or flue gas or the removal of solids from liquid wastes. In other problems,
particles are deliberately suspended in fluids to obtain separations of the particles into fractions
differing in size or density. The fluid is then recovered, sometimes for reuse. from the
fractionated particles.
The rate of sedimentation of a suspension of fine particles is difficult to predict because of the
large number of factors involved. The flocculation of a suspension is usually completed quite
rapidly so that it is not possible to detect an increase in the sedimentation rate in the early stages
after the formation of the suspension. Most fine suspensions flocculate readily in tap water and
it is generally necessary to add a deflocculating agent to maintain the particles individually
dispersed. A further factor influencing the sedimentation rate is the degree of agitation of the
suspension. Gentle stirring may produce accelerated settling if the suspension behaves as a
non-Newtonian fluid in which the apparent viscosity is a function of the rate of shear. A number
of empirical equations have been obtained for the rate of sedimentation of suspensions, as a
result of tests carried out in vertical tubes. For a given solid and liquid, the main factors which
affect the process are the height of the suspension, the diameter of the containing vessel, and
the volumetric concentration
Flocculation
The tendency of the particulate phase of colloidal dispersions to aggregate is an important
physical property which finds practical application in solid–liquid separation processes, such
as sedimentation and filtration. The aggregation of colloids is known as coagulation, or
flocculation. Particles dispersed in liquid media collide due to their relative motion; and the
stability (that is stability against aggregation) of the dispersion is determined by the interaction
between particles during these collisions. Attractive and repulsive forces can be operative
between the particles; these forces may react in different ways depending on environmental
conditions, such as salt concentration and pH. The commonly occurring forces between
colloidal particles are van der Waals forces, electrostatic forces and forces due to adsorbed
macromolecules. In the absence of macromolecules, aggregation is largely due to van der Waals
attractive forces, whereas stability is due to repulsive interaction between similarly charged
electrical double-layer. In a flocculated, or coagulated suspension the aggregates of fine
particles or flocs are the basic structural units and in a low shear rate process, such as gravity
sedimentation, their settling rates and sediment volumes depend largely on volumetric
concentration of floc and on interparticle forces.
Filtration
Filtration is the removal of solid particles from a fluid by passing the fluid through a filtering
medium on which the solids are deposited. The fluid may be a liquid or gas; the valuable stream
from the filter may be the fluid, or the solids or both. Sometimes it is neither, as when waste
solids must be separated from waste liquids prior to disposal. Usually, the feed is modified in
some way by pretreatment in order to increase the rate of filtration. This can be done by heating,
recrystallizing or adding ‘filter aid’ such as cellulose. As a result of the fact that there exists a
variety of materials to be filtered, there are many types of filters available.
Areas of application of the filtration process include;
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Water purification
Ore handling in the mining industry
Coatings and paint manufacturing
Chemical process, etc
In most industrial applications, it is the solids that are required during filtration and their
physical size and properties are of paramount importance. Therefore, the main factors to be
considered when selecting equipments and operating conditions for such facilities are:
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The properties of the fluid, particularly its viscosity, density and corrosive properties
The nature of the solid-its particle size, shape size distribution and packing
characteristics
The concentration of solids in suspension
The quantity of material to be handled and its value
Whether the feed liquor requires heating
Whether it is necessary to wash filtered solids
Filtration occurs by virtue of a pressure differential across the filtering medium. Filters are
classified into those that operate with a pressure above atmospheric pressure on the upstream
side of the filter medium and those that operate with atmospheric pressure on the upstream side
and a vacuum on the downstream side. Most industrial filters are either pressure filters, vacuum
filters or centrifugal separators. They are also either continuous or discontinuous, depending
on whether the discharge of filtered solids is steady or intermittent. During much of the
operating cycle of a discontinuous filter, the flow of fluid through the device is continuous, but
must be interrupted intermittently to allow for discharge of the accumulated solids. In a
continuous filter on the other hand, the discharge of both solids and fluid is uninterrupted as
long as the equipment is in operation.
Filters are divided into three main groups and they are cake filters, clarifying filters and
crossflow filters.
Cake filters
Cake filters separate large amounts of solids as a cake of crystals or sludge. There usually exists
a provision for washing the cake and removal of some of the liquid from the solids before
discharge. At the initial stage of filtration using a cake filter, some solid particles enter the pores
of the medium and are immobilized, but soon others begin to collect on the surface of the
septum. At the end of this brief initial period, the cake of solids does the filtration not the
septum; a visible cake of appreciable thickness builds up on the surface and must be
periodically removed. As with other filters, cake filters may operate either with above
atmospheric pressure upstream from the filter medium or with vacuum applied downstream.
As such, cake filters are mainly of two types; pressure and vacuum filters.
Filter media
The support for the filter cake (septum) must meet the following requirement:
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It must retain the solids to be filtered, giving a reasonably clear filtrate
It must not plug or blind
It must be resistant chemically and strong enough physically to withstand the process
conditions
It must permit the cake formed to discharge cleanly and completely
It must not be too expensive
Washing filter cakes
In order to wash soluble materials that may be retained on the filter cake following filtration, a
solvent miscible with the filtrate may be used as a wash. Water is the most common wash liquid.
The rate of flow of the wash liquid and the volume of liquid needed to reduce the solute content
of the cake to a desired degree are important in the design. The washing of filter cakes is in
three parts.
During the first part of the wash period, the effluent consists majorly of the filtrate that was left
of the filter which is swept out by the first wash liquid without any dilution. Here the
concentration is nearly constant. This stage of washing, called displacement wash, is the ideal
method of washing a cake.
The second stage of washing is characterized by a rapid drop in concentration of the effluent.
In the third stage, the solute concentration in the effluent is low and the remaining solute is
slowly leached from the cake. If sufficient wash liquid is used, the residual solute in the cake
can be reduced to any desired point; but once it is acceptably low, any further washing should
be stopped when the value of the unrecovered solute is less than the cost of recovering it.
Clarifying filters
Clarifying filters remove small amounts of solids or liquid droplets to produce a clean gas or
sparkling clear liquids such as beverages. The solid particles are either trapped inside the filter
medium or on external surfaces. The particles are caught by surface forces and immobilized on
the surfaces or within the flow channels where they reduce the effective diameter of the
channels but usually do not block them completely.
Principles of Clarification
If the solid particles being removed completely plug the pores of the filter medium and the rate
of plugging is constant with time, the mechanism is known as direct sieving. This phenomenon
is rarely encountered. Much more commonly, the particles partially block the pores giving a
gradual reduction in pore size. This phenomenon is called standard blocking.
Cross-flow filters
In a cross-flow filter, the feed suspension flows under pressure at a fairly high velocity across
the filter medium which prevents the layer of solids formed on the surface of the medium from
building up.
Crossflow filtration is majorly of three types; Microfiltration, ultrafiltration and hyperfiltration.
Microfiltration is used for particles in the size range of 0.5 to 5 micrometer. Ultrafiltration
covers a wider range from 0.5 – 10-3µm. The term hyperfiltration (or reverse osmosis) is used
for separation of small molecules or ions.
The ideal membrane for crossflow filtration should have a high porosity and a narrow pore size
distribution, with the largest pores slightly smaller than the particles or molecules to be
retained.